1. Field
Example embodiments described herein relate to an image sensor and a method of manufacturing the same, and, for example, to an image sensor capable of reducing image lag and a method of manufacturing the same.
2. Description of the Related Art
In general, image sensors are semiconductor elements that convert an optical image into an electrical signal. Image sensors can be divided into CCDs (charge coupled devices) and CMOS image sensors.
A CCD is an element in which MOS capacitors may be packed closely and accumulate or transfer charge carriers. A CMOS image sensor may be made to have the same number of MOS transistors as pixels by a CMOS technique using a control circuit and a signal processing circuit as peripheral circuits and using a switching method of sequentially detecting output by using the MOS transistors.
A CMOS image sensor may be generally divided into an APS (active pixel sensor) region, a reset region, a source follower region, and/or a peripheral circuit region. A photodiode and/or a floating diffusion region may be located on both sides of a transfer gate electrode included in the APS region. The transfer gate electrode may transfer photocharge accumulated in the photodiode to the floating diffusion region.
When such an APS region is formed, n− ions may be implanted into a region of the transfer gate electrode adjacent to the photodiode at low concentration. Then, a PPD (pinned photodiode) may be formed on a surface of the photodiode so as to reduce dark current. At that time, a p− dopant may be implanted into the region of the transfer gate adjacent to the photodiode. Thus, the concentration of n− dopant implanted into the region of the transfer gate electrode adjacent to the photodiode may be lowered.
When the floating diffusion region is formed, an n+ dopant may be implanted into a region of the transfer gate electrode adjacent to the floating diffusion region in higher concentrations. Thus, the dopant may be implanted into regions of the transfer gate at varying concentrations.
These varying concentrations may cause Fermi level difference. As a result, the transfer gate may act as a barrier when photocharge generated in the photodiode are transferred to the floating diffusion region. Thus the photocharge may not be completely transferred from the photodiode to the floating diffusion region, resulting in image lag.